A couple thoughts.
White is a no brainer for the color.
I was interested to read about IR heat through the windows, but there was not a lot of mention of heat transfer through the roof ribs, tho some people posted around it various times, and one specifically addressed it by putting 3/4” insulation inside below the ribs. Another mentioned the hot weather busses that had an air gap for shade.
I find this both interesting and challenging.
The air gap method is a hot weather winner! The trouble is that it provides 0 benefit in cold (by itself).
Addressing the inside with 3/4” foam board is good, and it doesn’t kill a lot of headroom. But seems a shame to insulate the roof with foam board thickness that is greater than the gain because of the loss from heat transfer from the ribs. Adding 3/4” is far from the 2” between ribs. Hopefully that makes sense as less than ideal. Perhaps somewhere to the left of the decimal point, but if and how far we do not know.
Ideally the insulation would be above the roof so that there is negligible heat, or cold transfer (gain/loss). That way all the insulation inside would be less affected by heat or cold from exposure to sun or cold.
Lots of comments dealt with paint insulation (but paint is not thick enough to insulate as mentioned) and reflectivity, but the cold that creeps in through transfer seems like a big deal too. From an efficiency standpoint, but practically it may be cheaper and easier to overcome with a heater.
It seems that 1/2” of insulation above the steel roof would significantly reduce the transfer of heat to the ribs, while adding some protection from cold. And the more the greater.
I was a member of the Foamie camper group before this, and it seems like foam on the roof covered with (PMF poor man’s fiberglass) over two or 3 layers of 1/4” poly packing foam, would contour and glue down easily.
The foam has a decent enough R value, and it is available in 1/4” X 4’ x 50’ rolls.
I see issues, but they are overcome able and it is fun enough to think about and bounce off of the group.
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